During a drilling operation, drilling fluid (or “mud”) is continuously pumped down through the drill string, through the mud motor if directional drilling, out through the drill bit and then back up a borehole annulus to the surface. Often the mud is made up of clays, chemical additives and an oil or water base and performs several important functions. The primary function of the mud is to cool and lubricate the drill bit. Other functions of the mud are to carry drill cuttings back up out of the well and maintain a hydrostatic pressure which prevents pressurized fluids in the earth formation from blowing out through the well borehole. When the drill bit drills thru rock formations it may encounter hydrocarbons entrapped in that formation. Therefore the drilling fluid performs an important secondary function by carrying hydrocarbon information back to the surface about the nature of the formation being penetrated.
As the drilling fluid returns to the surface these liberated hydrocarbon gases are released by direct agitation with a gas trap and a gas sample is drawn by a gas chromatograph. By examining the quantity and type of gas released the petroleum geologist and/or mudlogger may determine how feasible it is to obtain oil and/or gas from the well. This information can indicate to the geologist and/or mudlogger that they are on the correct well path in targeting the zone of interest or give indications that they have drilled out of targeted zone of interest. The longer the time a drill bit has drilled out of a targeted zone the less productive pay zone will be achieved. Current day drilling rig rate of penetrations have increased significantly to previous years rate of penetration due to advancements in drill bit technology, mud motors and drill fluids. Even horizontal wells rates of penetration have greatly increased.
Current gas chromatograph equipment take a spot sample of the liberated gases, but require up to 5 minutes and analyze the sample. While the sample is being analyzed over this time period other gas samples go by unanalyzed resulting in missing valuable information about the formation. Previous gas chromatography have been an instrumental method used for the separation and identification of chemical compounds and gases, but are a slow process as every component of gas moves through the column at a different rate to be analyzed. The time cycle required to separate and then analyze the hydrocarbons can take up to 5 minutes for full breakdown (methane to octane). Newer high speed chromatographs can provide data on the hydrocarbon spectra (methane to Butanes) in around 1 minute, but do not provide any data on the heavier hydrocarbons (pentane to octane). These heavier hydrocarbons (pentane to octane) are required to calculate proper gas ratio formulas to predict the hydrocarbon state. Also, many current gas chromatographs require a carrier gas such as helium to burn the detector at a consistent heat level and may also require an operator to ensure the gas chromatograph operates properly adding more cost to drilling operations. When using a combustion type detector in a gas chromatograph any amount of nitrogen and/or carbon dioxide will give a false reading in a gas chromatograph affecting the hydrocarbon response.
U.S. Pat. No. 7,741,605 discloses a mud logging gas detector using NDIR gas detector technology. The gas detector used 4 NDIR channels based on custom filter ranges in the mid infrared range to provide two gas concentration readings, one for Methane and one labeled as Propane which includes the heavier hydrocarbon gases from Ethane and up. Due to the overlap between the absorption spectra of the hydrocarbon gases, NDIR sensors cannot provide good separation of the gas components.
US Publication 2011/0313670 uses a type of spectrometer in the near infrared (IR) region rather than the mid IR. US Publication 2011/0313670 monitors a near IR hydrocarbon absorption region around 1.55-1.85 μm. However, near IR absorption band is roughly 3 orders of magnitude less sensitive than the primary band in the mid IR. In this portion of the near IR spectrum only one absorption band is available. This reduces the spectral information available for use in the analysis of the gas composition compared to using a broader spectral region such as the 2.0 to 5.0 μm mid IR range.
Accordingly, there is a need for a more reliable apparatus for identifying constituents of sample gas from drilling mud in a much quicker time frame to keep up with the increased drilling rate of penetration. In particular, there is need to distinguish one or more hydrocarbon types containing one to eight carbon atoms, such as methane, ethane, propane, butane isomers (n-butane, iso-butane), pentane isomers (n-pentane, iso-pentane, neo-pentane), hexane isomers (hexane, methylpentanes, dimethylbutanes), heptanes, and octanes, and carbon dioxide gas carried by the drilling fluid returning from a well, and to provide an indication of how much of those gases are present, if any. There is a need to provide said gas data at a quicker interval then current industry gas chromatographs to be able to react most effectively to changing hydrocarbon gases and ratios and hence maximize targeted zones and well production.
Furthermore, it would be desirable if such apparatus were lighter and less bulky than prior apparatus and did not include the expense of requiring an operator skilled in spectroscopic analysis to be on the rig site.